The present invention relates generally to mobile radio communications systems, and more specifically to an adaptive channel assignment technique that assigns channels using data collected from distributed base stations.
In the conventional cellular mobile radio communications system in which the service area is divided into cells covering an area of several tens of kilometers in diameter, radio channels are permanently assigned to the cells for establishing communications with mobile stations. For saving channel resources, the same frequencies are assigned to different cells if interference between them is acceptable. To meet the growing needs of mobile communications, a microcellular system has been proposed. According to this system, the service area is divided into "microcells" of diameter of several hundreds meters to reuse the same frequencies by a greater number of microcells and base stations with antennas as high as traffic lights are located in the respective microcells. However, difficulty arises in deploying base stations at regular intervals due to the presence of obstacles in the line-of-sight propagation, and hence their locations are not necessarily optimum from the view point of wave propagation and a very complex pattern of reflections from surrounding structures is likely to result. Furthermore, the pattern of interferences between microcells is very difficult to predict due to changing environments, and must therefore be updated at intervals after installation.
An adaptive channel assignment scheme is described in "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communication Systems", Yukitsuna Furuya et al., Proceedings of Second Nordic Seminar on Digital Land Mobile Radio Communication, October 1986. According to this scheme, each base station has a carrier sense function and controls the priority of channels depending on their busy/idle status. If a channel is sensed as being idle by a given station, that channel is given highest priority to be used by that station. With high probability of chances other stations will detect this channel as being busy and decrease their priority of this channel. Channel priorities are regularly updated so that carrier sense errors are minimized.